Sorting system for serially processing records

ABSTRACT

A speed and memory control system and method for use with a sort accelerator having a rebound sorter and merger is disclosed. The speed and memory control system includes a variable length shift register which utilizes circulating RAM indexing, tag extraction lookahead features to speed up access of records, and merge lookahead and memory management features to provide quick and effective storage of records.

This application is a file wrapper continuation of application Ser. No.08/225,751 filed Apr. 11, 1994, now abandoned, which application is adivisional of our application Ser. No. 07/918,871 filed Jul. 22, 1992,which issued as U.S. Pat. No. 5,349,684 on Sep. 20, 1994, which is afile wrapper continuation of application Ser. No. 07/374,342 filed onJun. 30, 1989, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a sort accelerator and more particularly to asort accelerator in which a rebound sorter is used as a merger and withwhich large numbers of records can be rapidly and efficiently sorted.The accelerator of the invention operates independently of a hostcomputer and is highly reliable, while using a minimum number ofcomponents and being otherwise economically manufacturable.

2. Background of the Prior Art

Pipelined VLSI implementations of sorting algorithms are known. Arebound sorter developed by IBM includes a set of processing elementswhich can be visualized as forming a U-shaped pipeline with data flowingdown along a left side and up along a right side. The processingelements operate concurrently to speed up the sorting process. Eachelement effects a comparison of data from two records and, dependingupon the results of the comparison, effects either vertical movementsdown along the left side and up along the right side or horizontalexchanges such that "heavier" records are moved to the left anddownwardly while "lighter" records move to the right and upwardly toeventually exit the upper end of the right side in sorted order. Reboundsorters of this type have advantages in speeding up sorting operations,but the cost of the required hardware has precluded extensive use ofsuch sorters, particularly for sorting large numbers of records.

Other types of sorting systems have been proposed, using multipleprocessing elements. All known types of systems have limitations,however, particularly with respect to operating speed, hardware costs,stability and reliability.

SUMMARY OF THE INVENTION

Some aspects of the invention relate to the discovery and recognition ofsources of problems and limitations of prior art sorting arrangements.Sorters in which processing operations are performed concurrently by anumber of elements are advantageous in speeding up sorting operations,but are limited in that they only allow sorting of a limited number ofrecords. The aforementioned IBM sorter, for example, allows sorting of agroup of records containing a maximum of one record more than the totalnumber of sorting elements. Thus fifteen processing elements sort agroup which contains a maximum of sixteen records. For large sorts, theamount and cost of hardware using such a system would be prohibitive.

In the sort accelerator of the present invention, a rebound sorter isprovided in which a limited number of processing elements are operatedconcurrently to sort a relatively small number of records into a group.The same rebound sorter is then also used to perform merges of sortedgroups into a larger group containing all of the records of the smallergroups in sorted order. With the sort accelerator of the presentinvention, the same hardware that sorts sixteen data items, for example,is also used to perform a sixteen-way merge. As the number of records tobe sorted goes up, increasing numbers of processing elements are notrequired to accommodate a large sort.

A sort accelerator which incorporates the invention may typically beconnected to a host computer and a host memory through an address anddata bus. The sort accelerator operates independently of the hostprocessor to sort the records at a high speed and to store the sortedrecords in the host memory. The independent operation of the sortaccelerator enables the host computer to freely perform other operationsas the sorting and merging operations take place.

In a preferred embodiment, the sort accelerator includes an inputsection, an output section, a sort control section, and a sortingsection. The sort control section operates to control the input sectionto effect sequential feeding of groups of records to the sorting sectionand control the output section to effect storage of a plurality ofsorted groups, either in a local working memory or in a working portionof the memory of a host computer. The input, sorting and output sectionsare controlled from the sort control section to receive groups ofrecords in unsorted order and to then effect sorting and temporarystorage of such groups of records in sorted order. It then operates tomerge such groups of sorted records into one larger sorted group orstring.

The sorting section of the sort accelerator can in one time period sorta maximum of a certain number of records in either descending orascending order, the certain number being 16 in an illustratedembodiment. Once sorted groups of 16 have been created, those groups canbe merged through a series of operations into one large sorted string.Once 16 large sorted strings have been created, they are then mergedinto a longer string. This process is repeated, producing 16 timeslarger strings each pass until all records have been sorted. Withenhancements, this method sorts at a rate proportional to a value≦N logwwhere "N" is the number of records and "w" is one more than the numberof processing elements.

To accomplish such sort and merge functions, the sorting section orrebound sorter need contain only 15 processing elements. A processingelement compares key bytes of a record to determine order. Processingelements are connected together via two record storage elements. Thearrangement of processing elements and record storage elements can beviewed as a vertical column where new input records enter the topleft-hand side of the column and sorted records exit from the topright-hand side of the Column.

Records are sorted into groups of 16 and stored in a storage section orworkspace memory. Once a sufficient number of groups have been sorted,the merge operation begins. To merge, each record is tagged to indicatewhat sorted group it belongs to. For ascending sorts, the smallestrecord of each group is placed in the rebound sorter until the smallestrecord is pushed out of the rebound sorter. This record is the smallestrecord amongst all of the smallest records of each individual sortedgroup. The next record fed into the rebound sorter is chosensequentially from the group which previously contained the smallest ofall records. This new record must be either equal to or greater than thefirst smallest record since it comes from a sorted group. This procedurecontinues so that a new record is chosen from the group in which thelast output record came from. Once all the strings are exhausted, themerge is complete yielding one large group of sorted records.

To rapidly and efficiently accomplish the sort and merge functions ofthe present invention, a variety of features have been incorporated intothe present design. In cooperation with external hardware, data ischecked for corruption from the point where data enters the sortaccelerator until it returns to the host processor. Proper operation ofthe sort and merge features of the sort accelerator are also checked bycomparing the size of adjacent records to determine if they have beenproperly ordered once these records exit the enhanced rebound sorter.

Two methods of detecting the integrity of data as it passes through thesystem are used. In the first method, byte parity is used to detecterrors in data transmission. Eight bits of data are represented by a 9bit value. The extra bit carries a parity value calculated at the datasource. Record integrity is checked by re-calculating the parity valuefrom the transmitted data and by comparing the newly calculated valuewith the transmitted parity bit. An inequality indicates a transmissionerror.

The second method used to insure data integrity involves using a twolevel checksum scheme. The checksum is calculated over multiple bytes aseach record enters the rebound sorter. The checksum is laterre-calculated as records exit the enhanced rebound sorter and comparedto the previously calculated checksum. This comparison checks for twotypes of errors which can occur in the rebound sorter. The first type oferror results from a single bit failure in the data path (includingstorage element failure) and the second results from the improperlycontrolled swapping of bytes between records (processing element controlfailure).

The sort order checker examines records as they exit the enhancedrebound sorter for proper sorting. A special storage element acting as ashift register holds one record and in cooperation with comparison logicdetermines whether the sorted records have been properly ordered. Theoutput of the comparison logic indicates whether the rebound sorter hasimproperly sorted the records.

The sort order checker also provides a "tie" bit indicating that keys ofadjacent records are equal. User software can utilize the "tie" bit toperform sorts on key fields which exceed the capacity of the sortaccelerator or to assist the user in performing post-sort elimination ofduplicate records.

A sort accelerator constructed in accordance with the invention furtherincludes in a preferred embodiment, features which achieve fast andstable sorting of large numbers of records while minimizing hardwarerequirements. One particular feature is the pipeline control forperforming multiple sort/merge operations in a sort accelerator. Thepipeline control allows groups of records to be sorted without mixingrecords from different groups and without having to flush the reboundsorter with dummy records before a new group of records can be sorted bythe rebound sorter.

The pipeline control allows the sort accelerator to begin pushing a newgroup of records into the rebound sorter immediately after the lastrecord of the previous group has been examined by the first processingelement. Pipeline control is accomplished by a series of shift registersin parallel with the rebound sorter. Whenever a new group of recordsenters the rebound sorter, a boundary value is set in the first pipelineshift register indicating to the processing elements that a new set ofrecords has entered the rebound sorter. The boundary value is shiftedthrough the series of shift registers along with the first record as itpasses through the processing element. The value indicates to theprocessing elements that records following the boundary value must notbe compared with records from other groups. In this way, the reboundsorter can remain fully loaded with records, allowing the overlapping ofloading and unloading of records from any number of different groups ofrecords.

Another feature of the sort accelerator relates to the performance of astable sort. A stable sort is one where records with equal keys exit thesort accelerator in the same relative order as they entered. Stablesorting is required by applications where the order of records withequal keys has already been determined. Additional hardware has beenadded and algorithms have been modified to incorporate stable sorting inthe rebound sorter.

Stable sorting is implemented by setting bits which identify a record bythe group from which it came. This group designation forces records fromdifferent groups having equal keys to exit the rebound sorter in theorder in which they entered. An "order" bit causes records from the samegroup with equal keys to exit the rebound sorter in the order in whichthey entered.

Still another feature of the sort accelerator relates to a mergelookahead/memory management arrangement through which the requiredamount of workspace memory need only be approximately 10% greater thanthe amount of memory required to store the data. Not only does thisfeature provide for efficient use of memory, but it also provides linearexpansion of workspace memory, thereby simplifying host systemmanagement of the workspace. Optimal merging reduces the number of timesdata is processed, and consequently reduces the time it takes to performthe merge/sort algorithm.

Allocation of workspace storage is managed by the sort control section.The sort control section determines whether enough storage has beenallocated for the sorted data. In the preferred embodiment, an extendedmerge/sort algorithm controls the sequencing of sorts and merges throughthe use of an input and an output phase.

The input phase, consisting of sorts and merges, continues until all ofthe unsorted records that make up the input string have been processedby either sorting a group of records or merging groups of records. Onceall the input records have been either sorted or merged, the outputphase begins.

At this point there will be a number of sorted groups or stringsexisting in multiple regions of workspace storage. The output phaseconsists of a series of merges followed by a final merge to complete thesorting of the records.

To further increase the speed of the merge function of the sortaccelerator, the sort accelerator utilizes a string number or tagextraction lookahead feature. This feature of the present inventionutilizes the information produced by the sort control section whichindicates which input string a given record is from. This information isnecessary to allow the sort control section to deliver the next recordto the rebound sorter. As previously described, a proper merge requiresthat the next input record must come from the same group as the lastoutput record. If the string information is delivered to the sortcontrol section too late, the rebound sorter will stall while waitingfor the next input record. In most cases, the tag lookahead provides thetag information to the sort control section before the next sortedrecord exits the rebound sorter. This allows the next input record to bewaiting at the input to the rebound sorter so that no delay occurs inthe merge operation.

Tag lookahead logic determines the group of the smallest record (forascending sorts) as early as practical before it is actually output fromthe sorter. This allows the sort accelerator to begin accessing the nextinput record before the next record has been output. Using thistechnique, the inter-record delay of merges can be totally eliminatedexcept for very short records or in cases where the sort decision ismade very late in the record.

One additional feature of the sort accelerator is the use of circulatingRAM indexing to implement a variable length shift register used as therecord storage element. A RAM is used to hold record data as it passesbetween processing elements. In this method, the length of a one bitvariable length shift register is programmed to allow for varying recordsizes. As the bit is shifted through the variable length shift register,it enables each row of the RAM in sequential order, and the RAM eitherreads or writes data. Consequently, this feature implements a shiftregister in a way that allows the shift register to vary in length whileusing a minimal amount of logic. This method also avoids the use ofshift registers for the data which are slower and require more power tooperate.

Other features and advantages will become more fully apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a sort accelerator constructed inaccordance with the invention, shown connected to a host computer andhost memory;

FIG. 2 shows the format of a record which is developed from an inputrecord, for transmission to an enhanced rebound sorter of theaccelerator;

FIG. 3 is a schematic diagram showing a rebound sorter of theaccelerator of the invention in a simplified and generalized form andportions of associated storage and pipeline circuits;

FIGS. 4A and 4B provide an illustrative simplified example of the flowof data through a sorter such as depicted in FIG. 3;

FIG. 5 is a schematic diagram providing a more detailed showing of theconstruction of the enhanced rebound sorter and an associated storagecircuit used in the sort accelerator of FIG. 1;

FIG. 6 shows logic circuitry of a control circuit of a processingelement of the rebound sorter shown in FIG. 5;

FIG. 7 shows one form of RAM index circuit usable with the storagecircuitry shown in FIGS. 1 and 5;

FIG. 8 shows another form of RAM index circuit usable with the storagecircuitry shown in FIGS. 1 and 5;

FIG. 9 shows a portion of a record control circuit of the accelerator ofFIG. 1, operable for supplying record timing signals;

FIG. 10 shows another portion of the record control circuit of theaccelerator of FIG. 1, operable for supplying tag timing signals;

FIG. 11 shows details of a pipeline control circuit shown generally inFIG. 1 and FIG. 5;

FIG. 12 shows tag selection circuitry of a string selection logiccircuit of a sort sequencer of the accelerator of FIG. 1;

FIGS. 13 through 17 show various portions of tag lookahead logiccircuitry of the accelerator of FIG. 1;

FIG. 18 provides an illustrative example of an organization ofintermediate storage which is used by the sort accelerator of FIG. 1;

FIGS. 19-22 illustrate string number assignments used for obtainingoptimal merging operations;

FIG. 23 shows checksum logic circuitry of an interface of the enhancedrebound sorter of FIG. 1;

FIGS. 24 through 27 show portions of logic circuitry of a sort orderchecker of the interface of the enhanced rebound sorter of FIG. 1; and

FIG. 28 shows an additional portion of logic circuitry of the sort orderchecker, used for generating a signal indicating that two consecutiverecords have the same keys.

DESCRIPTION OF A PREFERRED EMBODIMENT

Reference numeral 10 generally designates a sort accelerator which isconstructed in accordance with the principles of the invention. The sortaccelerator 10 is shown connected to a host computer 11, a host memory12, and a workspace memory 18, through an address and data bus 13 and isarranged to receive command data from the host computer 11 to sortrecords which are stored in the host memory 12 and to store the sortedrecords in the memory 12.

The command data may include, for example, the start address in memory12 of one group of records to be sorted or the start addresses ofrecords to be merged, the length of the records in each group, thelength of a key field in each record, the desired mode of sorting, thenumber of records in the group, and the start address in memory 12 inwhich sorted records are to be stored. Upon receipt of such commanddata, the sort accelerator 10 operates independently of the hostprocessor to sort or merge the designated records at high speed and tostore the sorted records in the host memory 12. The host computer isfree to perform other operations after applying a command signal to thesort accelerator 10.

The illustrated sort accelerator 10 includes a local address and databus 14 which is connected through a system interface 15 to the host bus13. A local memory 16 which provides workspace is shown connected to thelocal bus 14. The local memory 16 is optional but working memory isrequired by the illustrated system. In place of or in addition to theillustrated local memory 16, a portion of the host memory 12 may be usedto provide working memory in place of that provided by the local memory16. The local bus 14 is connected to a rebound sorter 18 through aninterface 20 which connects to input section 21, an output section 22and a sort sequencer 34. As shown, input section 21 includes a paritychecker 23, a buffer 24, a LW (long word) to byte unpacker 25, a taginserter 26 and a check parity/checksum calculator 27. The outputsection 22 is connected to the sorter 18 through an order checker 29 andincludes a checksum calculator and parity generator 30, a byte to longword packer 31, long word buffers 32 and a parity check/generate circuit33.

The rebound sorter 18 and the interface 20 are connected to a sortsequencer 34 which receives command data sent from the host processor 11and which monitors and controls input, sorting, storing, merging andoutput operations as hereinafter described. Sort sequencer 34 includes aregister array 35 and an ALU 36 which is connected to an instructionregister 37 to operate from a microprogram stored in a ROM 38. The ALU37 operates to effect supervisory control of input, sorting, merging andoutput operations. Sort sequencer 34 also includes a sort sequence, taglookahead and miscellaneous control circuitry 39 which operates toselect addresses sent to the input section 21 for rapid fetching ofrecords from memory during sorting and merging operations and to performother functions.

The enhanced rebound sorter 18 is designed to effect simultaneousprocessing operations on a group of 16 records through examination ofthe keys of such records. It includes fifteen sections 40-54 in whichsuch processing operations are performed, the number of sections beingone less than the number of records of the group being processed. Eachof the sections 40-54 includes a processing element and associatedrecord storage elements. Features of the rebound sorter 18 relate to theimplementation of record storage elements through the use of a storagecircuit 56 which includes a RAM and read and write circuits and whichoperates under control of a RAM index circuit 58.

Further features relate to the provision of a pipeline control circuit60 for controlling loading and emptying of the enhanced rebound sorter18 so as to avoid delays, while preventing mixing of separate groups ofrecords. A record control circuit 62 applies timing signals to theprocessing elements of the sections 40-54 and to the pipeline controlcircuit 60.

Additional features relate to the use of the enhanced rebound sorter 18for both sorting and merging operations in a manner such as to achievefast sorting of large numbers of records while minimizing hardwarerequirements and achieving other advantages. A specific feature relatesto identifying the individual records during processing operations in amanner such as to facilitate stable and reliable sorting of records andmerging of sorted records as hereinafter described.

In operation of the sort accelerator as thus far described, command datais initially sent through the interface section to the sort sequencer34. The sort sequencer 34 develops and stores corresponding control datain the register array 35, including the starting address of a firstrecord of each group of records to be input, the length of the recordsof the group, the length of a key field of each record, the number ofrecords and the starting address and other data with respect to wheresorted records are to be stored. After the command data is sent throughthe sort sequencer 34, an operation is then initiated in which the inputsection 21 operates using the address circuit 23 to fetch record datafrom the host memory 12, followed by another operation by the inputsection 21 which then uses the byte inserter 25 to develop modifiedrecords. A continuous stream of modified records may be sent to therebound sorter 18, each modified record consisting of a certain evennumber of serial bytes.

Preferably, each modified record may have a form as shown in FIG. 2,including one or more leading key bytes K which enter the sorter firstand which are followed by a tag byte T (added by the byte inserter 25)and one or more data bytes D. A trailing pad byte P, as shown, is addedby the byte inserter 25 to each record if it is necessary to obtain aneven number of bytes in the record.

The insertion of the tag byte T is a feature providing a reliable andefficient way of identifying the input string from which a given recordcame and to facilitate rapid merging operations as hereinafterdiscussed. In accordance with another feature, each record includes an"order" bit which is used to insure a highly stable sorting operation asalso discussed hereinafter, the "order" bit being preferably included inthe tag byte.

In an input phase of operation, a sorting operation is performed inwhich a group of 16 records are shifted into the illustrated reboundsorter 18. Immediately upon shifting of the last record of a group intothe rebound sorter 18, the records of the group of 16 start to exit therebound sorter serially in sorted order. A sorted string of 16 recordsis thus formed which is sent by the output section 22 into a section ofthe workspace memory 16.

Other features relate to the use of the pipeline control circuit 60 toenhance the speed of operation. As each sorted group of 16 recordsstarts to exit the rebound sorter 18, another sorting operation isinitiated in which another group of 16 records is in effectsimultaneously shifted into the rebound sorter 18. The pipeline controlcircuit 60 is designed to permit this operation without mixing therecords of the two groups.

As a second string of 16 records is then sorted and sent by the outputsection 22 to be stored in the workspace memory 16, a third group of 16records is shifted into the rebound sorter 18, followed by a fourth andsubsequent groups of 16 records until 16 strings of 16 sorted recordseach are stored in the workspace memory 16. Then an "up-merge" operationis performed, as hereinafter discussed in detail, in which records fromthe 16 sorted strings of 16 records each are fetched in a certain orderand shifted into the rebound sorter 18 to produce a single string of 256sorted records which is stored in the workspace memory. A merge can bestarted immediately following the last record or a sort or anothermerge.

After a single string of 256 records is stored, another series ofsorting operations are performed in which another 16 strings of 16records each are stored in the workspace memory 16, followed by anotherup-merge operation in which a second string of 256 sorted records isproduced and stored. After producing and storing of up to 16 of suchstrings of 256 records each, still another up-merge operation may beperformed to produce and store a single string of 4096 records.

If the input string has more than 4096 records, additional sorting andup-merging operations are performed until the entire input string isprocessed. In the input phase, records may be sorted into a number ofstrings all containing 4096 records, except the final string which maycontain a lesser number depending upon the length of the input string.The number of such strings which may be produced is limited only by theavailable memory.

At the completion of the sorting and up-merging operations of the inputphase, an output phase is initiated in which merges are performed as maybe necessary to reduce the number of strings to 16 or less, followed bya final merge in which the sorted records are written directly to thehost memory 12 starting at the addresses designated in the initialcommand from the host computer 11. Still other relate to management ofthe storage in working or intermediate memory of strings to be mergedand to the manner of supplying stored strings from intermediate memoryto the rebound sorter 18 for merging.

The sort accelerator 10 also includes features which relate to checkingoperations, including parity and checksum checks which are so performedas to allow optimum sorting and merging operations while detectingprocessing errors and insuring integrity of the sorted records which areoutput from the sort accelerator 10.

Rebound Sorter with Pipeline Control (FIGS. 3-5)

The heart of the sort accelerator 10 is the rebound sorter 18 which isshown in a simplified form in FIG. 3. FIGS. 4A and 4B provide anillustrative simplified example of the flow of data through the sorter.A more detailed showing of the construction of the rebound sorter 18 isprovided by FIG. 5, individual circuits of the sorter 18 being shown inother figures of the drawings.

The sorter 18 operates to arrange records in an order according to thenumeric values assigned to keys of the records which may typically bebytes providing ASCII representations of names or numbers. The sortermay operate to sort the records in either ascending or descending order,as desired. In describing operation herein and to minimize confusion,the terms "larger" and "smaller" are used then referring to comparisonsof record keys. It will be understood that the actual key valuesassociated with "larger" and "smaller" keys will depend on whether thesort being performed is ascending or descending and the use herein ofthe terms "larger" and "smaller" should not be construed as a limitationon the invention.

Referring to FIG. 3, the sorter 18 is an N-1 element rebound sorterwhich is capable of effectively comparing N record keys. N is 16 in theillustrated embodiment in which N-1 processing elements are indicated asPE₀ -PE_(N-1) and are included in the sections 40-54 shown in FIG. 1.

Key comparisons are performed one byte at a time, with all of the N-1processing elements or operating in parallel. Comparisons of bytes ofkeys continues until bytes from keys being compared are unequal. At thatpoint, a decision is made as to which key is the larger, based on theunsigned binary value of the bytes. For ascending sorts, the larger keyis the one with the larger unsigned binary byte value. For descendingsorts, the larger key is the one with the smaller unsigned binary bytevalue.

The N-1 element rebound sorter can be viewed as a black box with thefollowing characteristics. After being loaded with N-1 records, whenevera new record is put into the box, the smallest of the new record and N-1records already in the box emerges from the box.

As shown schematically in FIG. 3, a rebound sorter consists of theaforementioned N-1 processing elements or connected via inter-elementrecord storage sections each designated as an IERS, two IERS's beingassociated with each PE. Each PE accepts 2 input bytes and produces 2output bytes every time period. Each IERS accepts 1 byte and produces 1byte every time period. Each IERS holds half of a record and operates asa shift register. A byte input to the IERS is produced at the output R/2time periods later, where R is the number of bytes in a record. Thearrangement of processing elements and inter-element record storage canbe viewed as a vertical column where new records enter and exit thecolumn at the top left and right of FIG. 3.

The IERS elements are connected to the PEs in the following fashion:

a) The output byte from PEm going down the column is the input to IERSm.

b) The output of IERSm is the input to PEm+1 going down, except forIERSN-2 which is the input to IERSN+N-3. The input to the rebound sorteris the input to PE0.

c) The output byte from PEm going up the column is the input toIERSN-1+m-1, except for PE0 where it is the output of the sorter.

d) The output of IERSN-1+m-1 is the input to PEm-1.

Records with larger keys (those that sort later in the final sequence)fall toward the bottom of the column; while records with smaller keys(those that sort earlier in the final sequence) rise toward the top ofthe column. In the accelerator of the invention, groups of records to becompared are separated by creating a "boundary" condition which preventsnew records entering the column from being mixed with records already inthe column. This effectively flushes one group from the column whileloading a new group.

Each PE begins processing when the first byte of a record coming downand a record coming up the column are presented to it. If the recordcoming down is larger than the record coming up, it continues down thecolumn while the record coming up continues up the column. If the recordcoming down is smaller than the record coming up, it is sent back up thecolumn while the record coming up is sent back down the column. Sinceeach IERS contains only half a record, the even numbered PEs are half arecord out of phase with the odd numbered PEs.

The pipeline control of the accelerator of the invention controls thesequencing of the PEs so they know when new records are presented, whenthe end of the key is reached, and whether or not to compare them at all("boundary" condition).

In FIGS. 4A and 4B, a simplified three element rebound sorter is shownsorting a set of four 2-byte records, it being noted that theillustrated accelerator 10 requires a minimum of 4 bytes, two bytesbeing used in this example for simplicity. Consider the following inputsequence to the simplified rebound sorter as depicted, every 2 digitsbeing a record:

    85393274

In this example, the following assumptions are used:

a) The key is the entire record.

b) The boxes represent processing elements.

c) Vertical lines between the boxes represent inter-element recordstorage which will hold 1 digit.

d) The numbers shown next to the vertical lines represent the values ofthe data contained in the IERS. An "x" indicates "don't care" data.

e) The symbol "<-" will mark the first byte of a new group (a "boundary"condition).

f) PE states are indicated by "DS", "PV", "PH", "FV", and "xx" symbolsshown within the PE's:

g) DS=Deciding State (passing vertically)

h) PV=Passing vertically

i) PH=Passing horizontally

j) FV=Force vertical (passing vertically)

k) xx=Don't care.

The states shown are useful in understanding the operation of thesorter. For example, the initial state of the PE's is "FV" to load thedata into the IERS. After the 4th clock, the lowest PE element entersthe "DS" (deciding) state, and the first possible comparison between tworecords may occur.

After 14 clocks the final data value, 5, appears at the sorter output.The output stream is thus "32397485".

In FIG. 5, the processing elements PE0, PE1 and PEN-2 referred to in theforegoing discussion are contained in the sections 40, 41 and 54.

The processing element of the section 40 includes two multiplexers 65and 66, a control circuit 67 and a comparator 68. A "UL" input line 69at the upper left is connected to one input of the multiplexer 65 andone input of the multiplexer 66. A second "LR" input line 70 isconnected to a second input of the multiplexer 65 and a second input ofthe multiplexer 66. The outputs of the multiplexer 65 and 66 arerespectively connected to a lower left or "LL" output line 71 and anupper right or "UR" output line 72. The multiplexers 65 and 66 arecontrolled by a control circuit 67 which is controlled from the outputof the comparator 68, the inputs of comparator 68 being connected to the"UL" line 69 and the "LR" line 70.

When the processing element is operative to pass bytes vertically, themultiplexers 65 and 66 are so controlled that the bytes pass from the"UL" line 69 downwardly to the "LL" line 71 while bytes pass upwardlyfrom the "LR" line 70 to the "UR" line 72. In an opposite condition inwhich the bytes are passed horizontally, the bytes input on the "UL"line 69 are transmitted through multiplexer 66 to the "UR" line 72 whilethe bytes input on the "LR" line 70 are transmitted to the left throughmultiplexer 65 to the "LL" line 71. It should be understood that thelines 69-72 provide plural signal paths so as to transmit all bits of abyte in parallel, the bytes being transmitted one by one in byte-serialfashion.

All processing elements of the sections 40-54 perform the samefunctions. Each processing element receives the following informationduring each time period:

a) The next byte of the record coming down the column, applied throughthe "UL" line 69 of FIG. 5.

b) The next byte of the record coming up the column, applied through the"LR" line 70 of FIG. 5.

c) A boolean value which is true if the current bytes are the last bytesof the records, and false otherwise. (This value is termed "E/OREC" inFIG. 6 which is described hereinafter and which shows PE control logic;it is termed "EREC" for even numbered PE's and "OREC" for odd numberedPE's.)

d) A boolean value which is true if the current bytes are the firstbytes following the key, and false otherwise. (This value is termedE/OTAG" in FIG. 6 and is termed "ETAG" and "OTAG" for even and oddnumbered PE's.)

e) A boolean value which is true if the current records are being"forced to pass vertically", and false otherwise. To "pass vertically"means that the byte from the record coming down the column is to bepassed on down the column, while the byte from the record coming up thecolumn is to be passed on up the column. (This value is termed "FORCEV"in FIG. 6.)

f) A boolean value which is true for "ascending" sorts, and false for"descending" sorts. (This value is termed "ASCENDING" in FIG. 6.)

Each processing element maintains the following internal stateinformation:

a) A boolean value which is true if no decision has been made regardingwhich of the current records is "larger", and false otherwise.("DECIDING" in FIG. 6.)

b) A boolean value used after a decision has been made and which is trueif bytes are being "passed vertical", and false if bytes are being"passed horizontal". ("PASSV" in FIG. 6.)

Based on the information received and the current internal stateinformation, each processing element produces the following informationduring each time period:

a) The next byte to be passed down the column. This byte is passed downfrom the "LL" line 71 in FIG. 5.

b) The next byte to be passed up the column. This byte is passed up fromthe "UR" line 72 in FIG. 5.

c) New values for the internal state information.

The operation of a processing element can now be described simply, interms of the above information. If the processing element is beingforced to pass vertically, then all bytes of the record coming down arepassed down and the bytes of the record coming up are passed up,independent of the decision process below. This is used to load thesorter, empty it, and to separate groups of records to be sorted.

If the processing element is not being forced to pass vertically, thenthe following applies. If a decision has already been made, bytescontinue to be passed vertically or horizontally based on the decisionmade previously. Otherwise, the current input bytes are compared. Ifthey are unequal a decision is made at this time. If the byte coming upis larger than the byte coming down, then the byte coming up is passeddown while the byte coming down is passed up (pass horizontal).Otherwise, the byte coming up is passed up while the byte coming down ispassed down (pass vertical). When a decision is made, the processingelement remembers whether it passed vertically or horizontally, in orderto pass the remaining bytes of each record in the same direction.

The state of the ascending/descending input is used to determine thesense of larger and smaller in the comparison.

Each inter-element record storage section or IERS of FIG. 3 is avariable length shift register containing a half record of bytes. For arecord length of R, a byte input to an IERS at time period T, will beproduced at its output at time period T+R/2. This means that the shiftregister must be of length R/2. As aforementioned, the user record hasat least one byte added (the tag byte) and if the resultant length isodd, a pad byte is also added.

In the circuitry as shown in FIG. 5, each shift register is implementedby using RAM sections of the storage circuit 56 in combination withlatches which provide a stable interface to comparator logic. The firstshift register IERS0 of FIG. 3 is implemented by a RAM section 74 and aread-write circuit 76 which has a write input connected to the "LL"output line 71 and which has a read output connected through an outputlatch circuit 78 to the "UL" input line 69' of the next section 41. Theoutput latch circuit 78 forms an extra row of the shift register andinsures stable conditions for comparison of key bytes. Since the latchfor the extra row and at least 1 RAM row must be in the data path, theminimum size of the shift register is 2 rows. This sets the minimumrecord length for the rebound sorter at 4 bytes.

The RAM has a number of rows equal to half the maximum record size minus1, and a number of columns equal to 2 times the number of processingelements times 8. In the current implementation, the number ofprocessing elements is 15, making each row 2×15×8, or 240 bits wide.Since the maximum record size is 40, there are (40/2)-1, or 19 rows.This makes a total RAM size of 4560 bits.

In fashion similar to the implementation of the shift register IERS0 byRAM section 74 and read-write circuit 76, the shift register IERSN-1 ofFIG. 3 is implemented by a RAM section 80 and a read-write circuit 82which has a write input connected to the "UR" output line of the section41 and which has a read output connected through an output latch circuit84 connected to the line "LR" line 70 of the processing element ofsection 40. The output latch circuit 84, like the latch circuit 78,forms an extra row of the shift register.

The processing elements and shift registers of the sections 41 and 54illustrated in FIG. 5 are substantially identical to those of thesection 40, corresponding components being indicated by primed anddouble-primed numbers. The section 41 is an exception, in that it isshown as including a tag lookahead circuit 100, operative to avoidprocessing interruptions in a manner as hereinafter described inconnection with FIGS. 13-17. The row enables of the RAM sections 74 and80 and those for the other sections 41-54 of the sorter 18 are drivenfrom a RAM index circuit, two circuits 58 and 58' of different formsbeing shown in FIGS. 7 and 8, as described hereinafter.

The sections 41-54 of the sorter 18 are connected to the pipelinecontrol circuit 60 which includes elements arranged to store boundaryindications, such indications being shifted from one element to anotherin synchronism with the shifting of records in the sorter 18. Certain ofthe elements apply "FORCEV" signals to the control circuits of theprocessing elements to cause the processing elements to shift only inthe vertical direction during certain conditions and to avoid mixing ofrecords of one group with records of another group.

The pipeline elements which apply the "FORCEV" signals to the sections40, 41 and 54 are indicated by reference numerals 91, 92 and 93 in FIG.5, such elements being also operative for shifting of boundary conditionsignals. Additional pipeline elements are used solely for shifting ofboundary condition signals, the pipeline elements for the sections 54,41 and 40 being respectively indicated by reference numerals 94, 95 and96. The operation of the pipeline elements is described in detailhereinafter in connection with FIG. 11.

Processing Element Control Logic (FIG. 6)

FIG. 6 shows logic circuitry of the control circuit 67 of the section40, similar logic circuitry being included in the control circuits 67'and 67" and the control circuits of the other sections of the sorter 18.The aforementioned "FORCEV" signal is applied through a line 101 to oneinput of an OR gate 102 which develops an output on a "NEW PASS V" line103. A second input of the OR gate 102 is connected to the output of amultiplexer 104 which is controlled by a "DECIDING" signal on a line 105connected to the output of a latch 106. The latch 106 is controlledthrough an OR gate 108 either from a "E/OREC" signal on a line 109 orfrom a signal applied from the output of an AND gate 110 which has threeinputs.

One input of "AND" gate 110 is connected to the "DECIDING" line 105. Thesecond input is connected through a line 111 to an output of thecomparator circuit 68 at which a "UL=LR" signal is developed when thesignals at the upper left and lower right inputs 69 and 70 are equal.The third input of the AND gate is connected to a "-E/OTAG" line 112.

The multiplexer 104 has one input connected to a "PASS V" line 113 whichis connected to the output of a latch 114 to which the "NEW PASS V"signal on line 103 is applied. A second input of the multiplexer 104 isconnected to the output of an OR gate 116 which has one input connectedto the "UL=LR" line 111 and which has a second input connected to theoutput of a multiplexer 118. One input of the multiplexer 118 isconnected through a line 119 to a "UL>LR" signal line from thecomparator 68.

A second input is connected to a "UL<LR" signal applied on line 120 fromthe comparator circuit 68. The multiplexer 118 is controlled from theoutput of an OR gate 122 which has input lines 123 and 124 connected towhich "ASCENDING" and "E/OTAG" signals are applied. The operation of thecontrol logic of FIG. 6 will be apparent from the foregoing discussionof the operations performed by the PE elements.

As aforementioned, the row enables of the RAM sections 74 and 84 andthose for the other sections 41-54 of the sorter 18 are driven from aRAM index circuit, two circuits 58 and 58' of different forms beingshown in FIGS. 7 and 8.

RAM Index Circuit (FIG. 7)

The RAM index circuit 58 of FIG. 7 comprises a single decoder 131 and acounter 132. The counter is loaded through a count input line 133 with acount of U/2, counts down to 1 after being loaded with each count, andthen the operation is repeated. The output of the counter 132 is fedthrough the decoder 131 and through a multi-conductor output line 134 tothe RAM sections 74 and 84 for section 40 and to the corresponding RAMsections of the other sections 41-54. The outputs of the decoder 131then drive the RAM row enables, first for the read and then the writebefore advancing to the next row. The initial value at startup and afterthe counter reaches 1 (U/2), comes from the all but the low order bit ofthe user specified record size (divide by 2). It should be rememberedthat the user record has at least 1 byte added (the tag byte) and if theresultant length is odd, a pad byte is also added. This means that ifthe user record size is U, then U/2=(R/2)-1.

Alternate RAM Index Circuit (FIG. 8)

The RAM index circuit 58' of FIG. 8 reduces the the minimum cycle timeof the addressing logic at the expense of additional logic. The circuit58' drives the RAM row enables of the storage circuit 56 from a 1 bitvariable length shift register 136 which has a number of stagescorresponding to the maximum length of a half record with outputsconnected through a multi-conductor line 134' to the row enables of thestorage circuit 56. The stages of shift register 136 are configured in aring. The length of the shift register is configured to be half therecord length minus 1 through the application of initial control dataapplied through a multi-conductor line 133' to a decoder 138, suchcontrol data being applied after the record length has been determinedfrom control data supplied from the host processor. A single bitcirculates around the shift register ring providing row enable for firstthe read and then the write before advancing to the next row.

To achieve the operation of the shift register 136 as a ring, the inputof the decoder 138 is all but the low order bit of the user recordlength, which is half the record length input to the sorter 18 minusone. The outputs of the decoder are connected to the shift register,such that the exit end of the shift register is connected to the decoderoutput that is asserted when a 0 is input to the decoder. The next bitof the shift register (the one that shifts into bit 0) is connected tothe decoder output that is asserted when a 1 is input to the decoder,and so on.

Thus the output of decoder 138 loads a single bit into the shiftregister that will shift out of the shift register after N shifts, whereN is half the record length minus one. This is done at initializationand whenever the bit exits the shift register, thus forming the ring.Between shifts, the enabled row is first read and then written.

Record Control Circuit 62 (FIGS. 9 and 10)

FIGS. 9 and 10 show portions of the record control circuit 62. A portion62A shown in FIG. 9 is used primarily for record timing while a portion62B shown in FIG. 10 is used primarily for tag timing. These portionsproduce timing information for the processing elements and the pipelinecontrol. The record control circuits of FIGS. 9 and 10 receive thefollowing information during initialization:

a) Half the user record size, U/2, which is truncated toward 0 if recordsize is odd and which is applied on a line 141 in FIG. 9.

b) Key size, applied on a line 142 in FIG. 10.

The record control circuits of FIGS. 9 and 10 include components andmaintain internal state conditions and information, as follows:

a) A record byte counter, "RCTR" 143 in FIG. 9, which is initialized toU/2 and decremented every time period by a load/decrement signal derivedfrom the counter value (load at initialization or when RLTR=1, decrementotherwise). If decremented past zero, counter 143 is reset to U/2.

b) A tag byte counter, "TCTR" 146 in FIG. 10, which is initialized to Kthrough a signal which is applied through a multiplexer 148 controlledby a signal applied on a "INIT" line 149. The counter 146 is decrementedevery time period by the signal on line 144. If counter 146 is 0 whendecremented, it is set to U/2 through a signal applied from line 141through multiplexer 148.

c) A boolean value "RCNT" produced on a line 156 in FIG. 9 at the outputof a latch circuit 152 which has an input connected to the output of anexclusive OR circuit 153 having one input connected to a "RCTR=0" outputof counter 143 and a second input connected to line 150. The value"RCNT" indicates which half record is in progress in each PE. It isinitialized to false indicating that the first half of records areentering the even PEs, and the second half of records are entering theodd PEs. It changes whenever the record byte counter is decremented pastzero. If it is false it becomes true, and if it is true it becomesfalse.

d) A boolean value "TCNT" indicating whether the odd or even PE's willsee a tag byte next, produced on a line 156 in FIG. 10 at the output ofa latch circuit 157 which is connected to the output of an exclusive ORcircuit 158 having one input connected to a "TCTR=0" line at the outputof counter 146 and having a second input connected to the line 156.

Based on the internal state information, the record control produces thefollowing outputs:

a) A boolean value "EREC" indicating that the last byte of a record isbeing presented to the even processing elements, produced on a line 160which is connected to a latch circuit 161 at the output of an ANDcircuit 162 having inputs connected to the "RCNT" line 150 and to the"RCTR=0" output of the "RCTR" counter 143.

b) A boolean value "OREC" indicating that the last byte of a record isbeing presented to the odd processing elements, produced on a line 164which is connected to a latch circuit 165 at the output of an ANDcircuit 166 having inputs connected to the "RCTR=0" output of counter143 and to a "-RCNT" line 167 which is connected through an inverter 168to the "RCNT" line 150.

c) A boolean value "PCADV" indicating that the first byte of a record isbeing presented to either the even or odd processing elements, producedon a line 170 at the output of a latch circuit 172 connected to the"RCTR=0" output of the counter 143.

d) A boolean value "ETAG" indicating that the first byte following therecord key, i.e. the "tag" byte, is being presented to the evenprocessing elements, produced on a line 174 which is connected to alatch circuit 175 at the output of an AND circuit 176 having inputsconnected to the "TCNT" line 156 and to the "TCTR=0" output of the"TCTR" counter 146.

e) A boolean value "OTAG" indicating that the first byte following therecord key is being presented to the odd processing elements, producedon a line 178 which is connected to a latch circuit 179 at the output ofan AND circuit 180 having inputs connected to the "TCTR=0" output ofcounter 146 and to a "-TCNT" line 181 which is connected through aninverter 182 to the "TCNT" line 150.

Pipeline Control Circuit 60 (FIG. 11)

Very important features relate to the pipeline control circuit 60 whichallows the rebound sorter to be loaded and emptied, and preventsseparate groups of records from becoming mixed.

Circuitry of the pipeline elements 91-94 and 96 of the circuit 60 areshown in FIG. 11. The pipeline element 91, which is associated with theprocessing element of the first section 40 of the sorter 18, includes aninput line 186 to which a signal is applied in synchronized relation tothe input of a new record to the sorter. The input line 186 is connectedto one input of a multiplexer 188 having an output connected to a latch189, the output of the latch being connected to a line 190 which isconnected to a second input of the multiplexer 188.

A "FORCEV0" signal is developed on a line 192 at the output of a latch193 which is connected to the output of a multiplexer 194, one input ofthe multiplexer 194 being connected to the line 192 and a second inputthereof being connected to the output of an OR gate 196. Inputs of theOR gate 196 are connected to the lines 186 and 190 and into a "FORCEV1"line 192' of the next-following stage.

The multiplexers 188 and 194 are controlled by the "PCADV" signalapplied through line 170 from the record control circuit 162A of FIG. 9.The circuits of all other pipeline elements associated with the sorterprocessing elements are substantially identical to that of the element91. Except that line 186 comes from "BNDm-1" and in 93 there is no"FORCEVm+1" input.

The circuits of the pipeline elements 94 and 96 are also shown in FIG.11. The element 94 includes a multiplexer 198 and latch 199 whichcorrespond to the multiplexer 188 and latch 189 of the element 91.Similarly, the element 96 includes a multiplexer 198' and a latch 199',and a final output signal "NEWSTREAMOUT" being developed on a line 200.

In operation, the pipeline control circuit 60 receives the followinginformation during each time period:

a) The boolean value "PCADV" from circuit portions 62A (FIG. 9) whichindicates that the first byte of a record is being presented to eitherthe even or odd processing elements.

b) A boolean value "NEWSTREAM" applied on a line 186, indicating thatthe current byte being presented to the rebound sorter starts a newgroup of records to be sorted separately from those already in therebound sorter.

The pipeline control circuit 60 maintains the following internal stateinformation:

a) A boolean value "BNDO" through BNDN-2" for each PE produced at theline 190 of element 91 and its corresponding lines of the otherelements. This value indicates that a "boundary" between record groupshas come "down" the rebound sorter to that PE since the last time thefirst byte of a record was presented to either the even or oddprocessing elements.

b) The boolean value "BNDN-1" indicates that a "boundary" between recordgroups has moved from IERS "N-2" into IERS "N+N-3" since the last timethe first byte of a record was presented to either the even or oddprocessing elements.

c) Boolean values for processing elements "PEN-2" through "PE1",produced by elements following 94 thru 96 shown in FIG. 11, each suchvalue indicates that a "boundary" between record groups has come back"up" the rebound sorter to that PE since the last time the first byte ofa record was presented to either the even or odd processing elements.

d) A boolean value ("FORCEV0" through "FORCEVN-2", FIG. 11) for eachprocessing element indicating that that processing element is being"forced to pass vertically".

Based on the information received and the current internal state, thepipeline control produces the following information during each timeperiod:

a) A boolean value ("FORCEVO" THROUGH "FORCEVN-2" for each processingelement indicating that that processing element should "forcevertically".

b) The boolean value "NEWSTREAMOUT" on line 200 indicating that thecurrent record byte being output by the rebound sorter is the first byteof a new group of records being sorted. This is used by the sortorderchecker to prevent comparison of records from different groups.

The pipeline control updates its internal state information every timethe first byte of a record is presented to either the even or oddprocessing elements ("PCADV", every half record).

The "boundary" value enters the rebound sorter at the top, moves downthe left side, across the bottom, and up the right side, in parallelwith the records.

The new "force vertically" value for PE "N" is set if any of thefollowing is true:

"Boundary" is set for PE "N-1", or, for PEO, a "boundary" is enteringthe IERS("NEWSTREAMIN").

"Boundary" is set for PE "N".

"Force vertically" is set for PE "N+1". This does not apply to the lastPE.

This operation causes all PEs at or above a boundary to "forcevertically". It prevents mixing of records on different sides of aboundary, but allows records on the same side of a boundary to besorted.

Sort Sequencer 34 (FIG. 1)

As previously indicated, the sorter 18 with no internal boundaries canbe thought of as a "magic sorting box", such that a record is pushedinto the box, the smallest of all the records in the box (including therecord pushed in) pops out.

The sorting and merging operations of the accelerator 10 will be bestunderstood by recognizing and analyzing the problem of having N sortedstrings of data to be merged into one large sorted string, using an N-1element rebound sorter such as the sorter 18. Consider the followingalgorithm:

1) Set a boundary within the sorter 18 to start a new sort operation asdescribed hereinabove in connection with FIG. 11.

2) Push the first (smallest) record of the first N-1 strings into thesorter 18.

3) Push the first (smallest) record of the N'th string into the sorter18. The smallest of all the first records will pop out; this is thefirst (smallest) record of the merged string.

4) Identify the string from which the popped-out record came, and pushthe next record from that string into the sorter 18. The record thatpops out is the smallest record remaining in any of the input strings,and thus is the next record of the merged/sorted string.

5) Keep repeating the previous step. When the last record of any inputstring pops out of the sorter 18, push in an artificial record (which islarger than any "real" record) as the "next" record of that string. Whenthe first artificial record pops out of the sorter 18, the merge iscomplete, because no smaller (i.e. "real") records remain in the sorter18.

The output of the sorter 18 when operated under this algorithm will bethe merge of the N input strings; this can be proven by noting that:

a) Each record output from the sorter 18 is the smallest of all recordscurrently in the sorter.

b) Since the record being output is replaced by the next record of thesame input stream, which is not smaller than the output record since theinput stream is sorted, the next record output from the sorter will notbe smaller than the current record output from the sorter; i.e. therecords output from the sorter are in sorted order.

c) All records of all input streams will eventually be output from thesorter.

In order to perform merges using the above algorithm, it is desirable toknow which string a given record in the sorter came from. It is alsodesirable to know how to find the next record in that string and how todetermine whether there are any remaining records in that string. Thefollowing sections set forth a preferred arrangement for obtaining andusing the desired information. It should be noted that in place ofartificial records, the invention uses the lowest numbered non-emptystring until all strings are empty at which time NEWSTREAMIN isasserted.

Tag Insertion and Removal (FIGS. 1 & 2)

The illustrated sort accelerator keeps track of which input string agiven record in the sorter came from by using the byte inserter 25 ofthe input section 21 to insert a tag byte into each record as it isbeing fed into the sorter. As aforementioned in connection with FIG. 2,the tag byte is preferably inserted in a position immediately followingthe last byte of the key field and preceding the first byte of the datafield. The tag byte of a record contains an index that describes whichinput string of this merge operation contained the record. An 8-bit tagbyte would allow up to 256 strings to be input to a merge; actually, onebit of this byte is used to help implement and insure a stable sortingoperation as hereinafter described, so that a maximum of 128 strings maybe merged. The illustrated embodiment is limited to merging 16 stringsat a time by the number of processing elements in the sorter (15),requiring only 4 bits for the string identification.

In an output operation in which a string of completely sorted records isoutput by the output section 22, for transfer to the host memory or fortemporary storage in the local memory 16, the data packer 31 of theoutput section 22 removes the tags previously inserted by the inputsection 21 from the records as they are output from the sorter, so thatthe tags do not occupy any space in storage. As discussed hereinafter,the user may optionally specify that tags should be left in the recordswhen the records are output to the user's buffer by the final mergepass.

As records are output from the sorter, the tag byte is examined todetermine the input string from which the record came. This stringnumber is fed into the string selection logic circuit 39 of the sortsequencer 34, and the sort sequencer 34 is controlled by microcode inROM 38 to use the output of the string selection logic circuit 39, todetermine the address of the next record to input to the sorter; thatrecord address, along with the string number produced by the stringselection logic circuit 39, is given to the input section 22 to startreading the next record.

String Selection Logic Circuit 39 (FIG. 12)

In general, the string selected to provide the next input record is thesame as the string whose index was in the tag of the current outputrecord. The exception is when the prospective string is exhausted. Thetechnique of using an artificial, large record in place of the nextrecord from that string, used in the algorithm described hereinabove,works but is clumsy and time consuming; it is found that it is much moreefficient (and just as correct) to pick any string that is not exhaustedand input a record from that string (in order to preserve sortstability, now the lowest non-empty string must be selected). Thisoperation is performed by the string selection logic circuit 39.

The string selection logic circuit 39 operates using a file of validbits containing one bit for each input string and using circuitry toperform functions as now described. A portion of such circuitry is shownin FIG. 12 which is described hereinafter in connection with theoperation of tag lookahead logic shown in FIGS. 13-17. The file of validbits is set to all zeros at the beginning of a merge operation to showthat none of the input strings has been exhausted. The tag from theoutput record indexes into the file of valid bits to see if thedesignated string is exhausted (i.e. the bit is a one); if not, thatstring number is output to the sort sequencer. If the designated stringis exhausted a priority encode is performed on the file of valid bits tofind the lowest-numbered string that is not exhausted, and the number ofthis string is sent to the sort sequencer. If all input strings areexhausted a flag is set which informs the sort sequencer of this fact.

The sort sequencer 34 contains two files of registers in the registerarray 35 to maintain the current state of a merge operation; one filecontains a next record address for each string, and the other filecontains the final record address in each string. Upon obtaining astring index from the string selection logic, the sort sequencer fetchesthe next record address for that string from the first file and sends itto the input section 21; it also updates the next record address forthat string. When the address of the last record in a string has beensent to the input section 21 (as determined by comparing the next recordaddress to the final record address for that string in the secondregister file), the sort sequencer sets the valid bit corresponding tothat string in the string selection logic's bit file to indicate thatthe string is now exhausted.

The string selection logic circuit 39 is also used when the number ofstrings to be merged is less than 16. The algorithm presented above onlyworks when the number of strings is one greater than the number ofelements in the sorter. When there are fewer strings to be merged, anumber of "null" strings (containing no records) must be added to themerge to bring the number of strings to the correct value. The stringselection logic circuit 39 is used to insure that an attempt toinitially load the sorter with a record from one of the "null" strings(as described in the first step of the algorithm described above) willactually cause a record from one of the "real" strings to be loaded.This is done by initializing the valid bit of the "null" strings to aone instead of a zero in the string selection logic circuit 39 at thebeginning of a merge operation.

Tag Lookahead and Associated Logic (FIGS. 12-17)

Even with specialized string selection logic and a fast sort sequencer,there is a delay between the output of the tag for the current recordand the delivery of the address of the next input record to the InputProcessor. The input section 21 must fetch the first bytes of the newrecord before they can be input to the sorter. This can add up to anoticeable delay between records, which causes the input section 21 tostall the sorter waiting for data and the merge process to slow down.

To eliminate this delay, the sort accelerator implements tag lookaheadlogic in order to determine the tag of the smallest record before it isactually output from the sorter.

The tag lookahead logic operates in conjunction with string selectorlogic which is shown in FIG. 12 and which is operative to select fromamong three possible values for the tag of the next record to be appliedto the sorter 18. These three values are a "PE1TAG" developed on a line202 by logic circuitry of FIG. 17 in a manner as hereinafter described,the tag "ITAG" of an input record, applied on a line 203 and the tag ofthe lowest numbered stream with tags remaining, developed on a line 204by a priority encoder 206 to which status data are applied through aline 207 (described hereinabove).

A multiplexer 208 is operated by a "PV0" control signal applied on aline 209 to develop a "WINTAG" signal on a line 210 from the "ITAG" and"PEITAG" signals. If "PV0" is true, "PE1TAG" is selected; otherwise,"ITAG" is selected. Then the status of the "WINTAG" stream on line 210is examined by a stream status lookup section 212 which develops anoutput signal on a line 213, applied to a multiplexer 214 to which the"WINTAG" and "ALTTAG" signals are applied. If the "WINTAG" stream is notempty, then "WINTAG" is used as "NEXTTAG" developed on an output line216 of the multiplexer 214. Otherwise, the lowest numbered stream thatis not empty ("ALTTAG") is used as "NEXTTAG" on line 216.

The tag lookahead logic circuit 100 includes circuitry shown in FIG. 13which is used in developing the "PE1TAG" signal on line 202 and also abackup tag "BTAG" on a line 218; logic circuitry shown in FIG. 14 whichis used in asserting a "DECISION" signal on a line 220 when PE0 hasdecided on the current record; logic circuitry shown in FIG. 15 fordeveloping a "VALTAG" signal on a line 222 to validate a record exitingPE0; logic circuitry shown in FIG. 16 for developing a "ADVTAG" signalon a line 224 to advance a tag from the "BTAG" line 218 to the circuitof FIG. 15 to the "PE1TAG" line 202; and circuitry shown in FIG. 17which develops a "TAGSEEN" signal on a line 226 when a tag has beenfound in the last half of a current record exiting PE0.

With respect to the operation of the tag lookahead logic circuits, it isnoted that the record to be output from the sorter always comes fromPE0. The inputs of PE0 are the record being input to the sorter and theoutput of IERS N-1, which is fed by the upper output of PE1. The tag ofthe input record is known; the tag of the record in PE1 is provided tothe tag selection logic 39 by special logic in PE1 ("PE1TAG" on line 202in FIGS. 12 and 13). The tag lookahead logic monitors the internal stateof PE0 watching for a decision to be made ("DECISION" on line 220 inFIG. 14); when it is made, the "Pass Vertical" state bit for PE0 ("PV0"in FIG. 12) is examined to see which record will be the one to exit PE0.The tag of this record ("WINTAG" in FIG. 12) is then used as the inputto the string selection logic (FIG. 12), thereby getting a jump of up tohalf of a record on the actual output. Using this technique theinter-record delay on merges can be totally eliminated except for verysmall records or when the "Decision" is made very late in the record.

The logic circuitry of FIG. 13 includes a multiplexer 228 which isoperated by an inversion of the "ADVTAG" signal on line 224 from thecircuit of FIG. 16 and which has one input connected to a latch circuit229 which is connected to the output line 202. A second input of themultiplexer 228 is connected to the output of a multiplexer 230 which isoperated by the "OTAG" signal on line 178 from the record controlcircuit 62B of FIG. 10.

One input of the multiplexer 230 is connected to a "UR1" on line 231while a second input thereof is connected to the "BTAG" on line 218.Line 218 is connected to the output of a latch 232 which is driven froma multiplexer 234, controlled from the line 178 and having inputsconnected to the lines 218 and 231.

The tag for the record exiting PE1 ("PE1TAG" on line 202 of FIG. 13), isextracted as follows: If not advancing tag ("-ADVTAG"), then theprevious value of "PE1TAG" is latched. If advancing tag ("-ADVTAG") online 224 is true and a tag is exiting PE1 ("OTAG" on line 178), thenthat tag is selected through ("UR1") on line 231. If a tag is advancingand a tag is not exiting PE1, then the backup tag ("BTAG" on line 218)is selected, the backup tag being latched by circuit 232 whenever a tagexits PE1 ("OTAG").

Referring to FIG. 14, the "DECISION" signal on line 220 is developed byan AND gate 236 which has one input connected to a "DECENB" line 237 anda second input connected to the output of an AND circuit 238 which hasinputs connected to a "-DECIDING0" line 239 and to the "VALTAG" line 220from the circuit of FIG. 15.

Referring to FIG. 15, the "VALTAG" signal on line 220 is developed atthe output of a latch 240 coupled to the output of an OR circuit 242having inputs connected to three AND gates 243, 244 and 245. The inputsof gates 243, 244 and 245 are connected to lines 220 and 226, to a"-EREC" line 246 derived from an inversion of the "EREC" line 160 ofFIG. 9 and to other signals derived from the record control circuits ofFIGS. 9 and 10, as described hereinabove.

The "VALTAG" signal on line 220 validates the tag for the record exitingPE1 ("PE1TAG") if it is the tag for the record exiting "PE0". "VALTAG"becomes deasserted whenever the last byte of a record exits PE0 ("-EREC"and "VALTAG"). If the tag is in the first half of the record, "VALTAG"becomes asserted when a record starts entering PE0 ("EREC" and"TAGSEEN"). If the tag is in the second half of the record, "VALTAG"becomes asserted as soon as the tag exits PE1 ("TCTR=0" and "TCNT" and"-RCNT").

With regard to the circuit of FIG. 16, the output line 224 is connectedto a latch 250 which is connected to the output of an OR gate 252 havinginputs connected to two AND gates, AND gate 253 having inputs connectedto lines 160 and 226 and AND gate 254 having inputs connected to lines146, 156 and 167 from the circuits of FIGS. 9 and 10.

A tag is advanced from "BTAG" to "PE1TAG" by "ADVTAG". If the tag is inthe first half of the record, "ADVTAG" is asserted during the first byteof a record presented to PE0 ("EREC" and "TAGSEEN"). If the tag is inthe second half of the record, "ADVTAG" is asserted while the tag isexiting PE1 ("TCTR=0" and "TCNT" and "-RCNT").

FIG. 17 shows logic which keeps track of whether the tag has been seenduring the last half of the current record exiting PE0. It includes alatch 256 which is connected to the output of an AND gate 258 having oneinput connected to the "-EREC" line 246. The second input of gate 258 isconnected to the output of an OR gate 259 having one input connected tothe line 226 and having a second input connected to an AND gate 260which has inputs connected to the lines 150 and 156 from the recordcontrol circuits of FIGS. 9 and 10. "TAGSEEN" on line 226 becomesdeasserted whenever the last byte of a record exits PE0 ("-EREC")."TAGSEEN" becomes asserted when the tag exits PE1 during the first halfof a record ("TCTR=0" and "TCNT" and "RCNT").

Sort Sequencer 34 (FIGS. 1 & 18-22)

Whereas the other areas of the sort accelerator operate on the datastrings to be processed, the sort sequencer calculates the externalmemory addresses and provides overall control of the sort accelerator.

The illustrated preferred embodiment of a sort accelerator uses the16-way enhanced rebound sorter 18 as shown, but the following sectionshave been generalized to the use of an N-way sorter having functionalcharacteristics equivalent to those of the sorter 18 as disclosed.

As aforementioned, the sort sequencer 34 has a control processor whichincludes the register array 36 and the flag register 37. The registerarray 36 may preferably be an array of 64 32-bit registers in four banksand the flag register may be a 32 bit register. The control processoralso includes the ALU 37 which is preferably a 32-bit ALU. In apractical embodiment, this architecture is capable of reading oneregister, performing an ALU operation, and writing the same register inone 133 ns clock.

The register array serves two purposes. The host processor communicatescontrol information to the sort accelerator by initializing theseregisters, and the control processor uses them as the variables of itsmicrocoded program. Addresses, counts, and status information aremaintained in the array. The array is organized as four banks of 16registers each. Each bank may be indexed with a variable, allowing thetag from merge operations to be quickly translated from a string indexinto an address pointing to the next record to be fetched.

A microprogram for the control processor is stored in the ROM 38 in anarray which may, for example, consist of 512 microwords of 60 bits, eachmicroword being divided up into fields controlling the source of ALUoperands, the ALU operation to be performed, the destination of ALUoperations, and the location of the next microword to be executed.Additional fields in the microword may enable the writing of data tovarious destinations and control the flow of microprogram execution andmicrocode subroutines may be supported. It will be understood that theinvention is not limited to the use of any particular form of controlprocessor for performing the sorting operations as hereinafter describedin detail and as depicted in part in FIGS. 18-22. FIG. 18 illustratesthe organization of intermediate storage which is important to anunderstanding of the operation of the sort sequencer and FIGS. 19-22illustrate sort sequencer modifications which are important in achievingsort stability.

Organization of Intermediate Storage (FIG. 18)

The algorithm used by the sort accelerator 10 requires intermediatestorage which is organized into regions of different sizes. The smallestregion is located at the base of the intermediate storage area and isused to hold up to N strings of N records. The second region is locatedat the end of the first region and is used to hold up to N strings ofN×N records. Each successive region is larger than its predecessor by afactor of N. FIG. 18 provides an illustrative example of theorganization which would apply if N were 4, rather than 16 as in theillustrated embodiment. In this example, the first region, indicated byR0, stores four records, the second region, indicated by R1, stores fourstrings of sixteen records, i.e. it stores N×N records. The thirdregion, indicated by R2, with only one-fourth being shown stores fourstrings of sixty-four records, i.e. it stores N×N×N records.

In the illustrated embodiment in which N is sixteen, the first region R0stores sixteen strings of sixteen records (256 records), the secondregion R1 stores 4096 records and the third region R2 stores 65,536records. Subsequent regions contain 16 times the number of records inthe preceding region. Any number of additional regions may be provideddepending upon the available memory.

The location and size of the intermediate storage is programmed byloading the base address and end address of storage in the registerarray at operation initialization. If insufficient storage is allocatedfor an operation, the control processor will suspend the operation andinterrupt the host processor. The storage can be dynamically expanded byincreasing the end address.

Extended Merge-sort Algorithm

In the illustrated embodiment, an extended merge-sort algorithm isembedded in the microprogram stored in the ROM 38, but any equivalentmeans for implementing the algorithm may be used. The algorithm utilizesthe N-way merge capabilities of the enhanced rebound sorter to reorderan unlimited number of records. The algorithm consists of two phasescalled the input phase and the output phase. As will be discussed below,the sequence of sorts and merges which make up these phases is importantfor sort stability, memory efficiency, and performance.

Input Phase

The input phase, consisting of sorts and merges, is in effect until allof the unsorted records that make up the input string have beenprocessed. Each sort operates on N unordered records, creating a sortedstring of N records in region 0 of intermediate storage. Each mergeoperates on the N sorted strings of a filled region, creating a singlesorted string in the next region N times larger than each of the inputstrings.

The input phase begins by sorting the first N records from the inputstring into one string of N records in region 0. The next N records fromthe input string are then sorted into a second string of N records. Thisprocess continues until N strings of N records exist in region 0, atwhich point the sorting is suspended while they are merged into a singlestring of N×N records in region 1.

Once this "merge-up" is complete, the input phase resumes sorting theinput string. Eventually, N new strings of N records are created inregion 0, sorting is suspended, and they are merged into a second stringof N×N records in region 1. This process continues until there are NN×N-lengthed strings in region 1, at which point they are merged into asingle string of N×N×N recorded in region 2. The number of regions islimited by the amount of intermediate storage available.

The input phase continues, performing a "merge-up" whenever possible,until the entire input string is processed. The number of records in theinput string is programmed into the register array when the operation isinitialized. There are several option bits in the command register, oneof which indicates whether or not the input string contains all of therecords to be sorted. If this bit, the END bit, is not set, the controlprocessor will interrupt the host processor before the output phasebegins, allowing the host processor to include another input string inthe same sort operation. The record count register is continuallydecremented as the input string is processed.

Output Phase

At the completion of the input phase, there will be a number of stringsexisting in multiple regions. The output phase consists of a series ofmerges to reduce this number to N or less, and a final merge directly tothe destination address.

At the beginning of the output phase, there will be up to N-1 fulllengthed strings in each region plus a partial string in region 0. Thepartial string consists of the total number of records being sortedmodule N (possibly 0) which exists because the total number of recordsneed not be a multiple of N. For sort stability purposes, priority isassigned from highest region to lowest. Within one region, priority islowest string to highest. This coincides with the order the strings werecreated.

The output phase proceeds in a very efficient and reliable manner.Starting with the lowest nonempty region, a merge is performed whichincludes as many regions as possible with the restriction that the totalnumber of strings involved is less than or equal to N. This mergecreates a new partial string in the first nonempty region not includedin the merge.

These multi-region merges continue until the second highest region isreached. At this point, if both second highest and highest regions canbe included, then they will participate in the final merge of the sortoperation which is written to the destination location. If the highestregion can not be included, then the second highest region is prohibitedfrom participating, with the output creating a partial merge at thesecond highest level.

The later case represents a performance optimization. Because the sizeof strings in the last two regions are significant with respect to thesize of the entire sort operation, the manipulation of these strings isminimized by merging just enough strings from the second highest regioninto a partial string at the highest region to leave exactly N strings.This is called the "optimal" merge. The N strings then participate inthe final merge.

Throughout the entire sort operation, the string count of each region ismaintained in one bank of the register array to allow indexing. Thelength of the partial string is maintained in another register in thearray. The lengths of the other strings are known from their regionnumbers.

Because there is only one register to maintain partial string lengths,only one partial string can exist at any time. The output phase includesthe partial string in every merge, and each of these merges creates alarger partial string in a higher region. The partial string lengthregister is continually updated to reflect the size of the new partialstring. The partial string also participates in the "optimal" merge.

Management of Intermediate Storage

The organization of intermediate storage is described above inconnection with FIG. 18. This section details the bookkeeping performedby the control processor during the input phase of a sort operation.

The control processor needs to determine if enough storage has beenallocated for the operation. To do this with minimal performancedegradation, two registers in the register array 35 are used to maintainthe highest active region number and the highest active address. At thebeginning of each sort or merge within the extended merge-sortalgorithm, the current region is compared to the highest active region.If it is less than the highest active region minus 1, then the sort ormerge continues normally; otherwise, the control processor is about tocreate a new string in the highest region and needs to verify thatenough storage has been allocated to proceed.

The verification begins by using the highest active address register todetermine if there is room for another full string in the next region.If there is, then the extended merge-sort proceeds normally; otherwise,the control processor determines how many strings from the currentregion will fit into a partial string at the next region, and uses thatinformation to determine how many additional records could be processedif these strings were merged. If this number is less than the number ofrecords remaining in the input string, then the control processorinterrupts the host processor indicating that more storage must beallocated to finish the operation.

If the number of additional records allowable is greater than the numberof records remaining, then the operation can be finished withoutadditional storage. If the END bit in the command register is set, thenthe control processor merges up just enough strings from this level suchthat the number of remaining strings in this level plus the number ofadditional strings in this level which will be formed from future inputplus the number of strings in the next level is identically N. A flag isset which will indicate to the output phase that an "optimal" merge hasbeen performed, and the length of this special partial string is savedin a register in the register array. This register is not the same asthe partial string length register used by the output phase.

If the END bit is not set, then an additional option bit in the commandregister, the LOCK bit, is examined. If the LOCK bit is set, then thecontrol processor merges as many strings as possible into the specialpartial string at the next level, sets the merge flag, saves the lengthof the special partial string, and continues creating strings at level0. If the LOCK bit is not set, then the host processor is interruptedallowing it to set the END bit, set the LOCK bit, or increase thestorage end address.

The "optimal" merge guarantees that the performance of the extendedmerge-sort is optimized. The "compaction" merge occurs because thecontrol processor does not know the total number of records in theoperation and the host processor has given permission to lock the sizeof the intermediate storage. Once locked, it is illegal to increase theend of storage address. This merge optimizes the number of records thatcan be sorted with the given amount of storage.

Sort Control

The sorting process of the input phase as described hereinbeforerequires that the sort accelerator read in N unordered records from theinput string, sort them, and place the results in a new string in region0 of intermediate storage.

The sort control may preferably exist within a single loop in themicrocode. First, the record count register is checked to see if theinput string has been exhausted. Once it is, control is passed to theoutput phase. If there are more input records, the source address in theregister array is passed to the Input Processor. This register, whichwas loaded with the address of the input string when the operation wasinitialized, is then incremented by the number of bytes per record.Another count register in the array is incremented by one, and the sortloop continues until the count reaches N.

Merge Control

Unlike the sort control which may preferably be handled purely inmicrocode, the microcode for the merge control is preferably acceleratedwith the string selection circuit 39, using the merge algorithm asdescribed hereinbefore. The string selection logic is also describedhereinabove. The sort sequencer is also responsible for supplying thetag which the Input Processor inserts into each record.

Sort Stability

A "stable" sort is one in which records with equal keys exit the sort inthe same relative order as that in which they entered. To keep thedesign simple, and reduce the additional storage and time requirements,use is made of the tag byte already required by the N-way mergealgorithm, to insure sort stability. This feature keeps the total recordsize relatively small and does not require extensive changes to the sortsequencer or the rebound sorter.

To ensure that this predetermined order is not changed, several featuresare included in the sort accelerator 10, using special hardware andalgorithms.

As has been described, a tag byte is inserted into the input recordstream following the key bytes, but preceding the remaining data bytes.The high order 4 bits of the tag byte are set to the string number fromwhich the record came (for a merge), or zero (for a sort).

The tag byte is always compared in the processing elements in"ascending" order. Placing the tag byte after the key, but before theremaining data allows the processing elements to decide which way topass the remaining non-key bytes before those bytes are presented. Thestring number will cause records from different strings with equal keysto exit the rebound sorter in the order in which they entered. The"order" bit will cause records from the same string with equal keys toexit the rebound sorter in the order in which they entered.

Sort Sequencer Enhancements

For a given merge, strings of records are selected that entered thesorting process together. That is to say, there are no records notselected that entered the sorting process before some records that areselected, but after others that are selected (they could potentiallybelong in this merge).

A unique number is selected for each string which indicates the relativeorder in which the records in that string entered the sorting processwith regard to the other strings within the same merge. Each record of astring has the string number inserted into the high order bits of thetag byte. If the order of 2 records from different strings has not beenresolved when their tag bytes are compared, the string number willinsure that they exit from the merge in the same order that theyentered.

As sorted strings are written into a region of workspace, they arewritten into the lowest available row in that region. Whenever a regionbecomes full, it is merged up into the lowest available row in the nexthigher region. From this you can see that in the absence of a partialmerge (described below), the highest region contains the strings thatentered the sort first, the second highest region second, and so on.Within a region, the lowest row contains the string that entered thesort first, the next row second, and so on.

In the descriptions below, string numbers are assigned sequentiallystarting with 0. Unless otherwise specified, within a group of rows thestring numbers are assigned sequentially from the lowest to the highestrow. As shown in FIG. 19, string numbers 0 through 15, in the columnunder "S" are in sequential order, string 15 being at the highestaddress "HA" and string 0 being at the lowest address "LA".

There are 4 different cases for assigning string numbers to the stringsinvolved in a merge:

1) When a region is filled and a full merge to the next higher region isperformed, the string numbers are assigned as a group to the 16 rows (asin FIG. 19).

2) When a partial merge is performed due to a restricted amount ofworkspace, not all rows of the second highest region are merged up intothe highest region. In this case, string numbers are assigned as a groupto as many of the highest numbered rows of the second highest regionthat will fit in the last row of the highest region. In the example ofFIG. 20, "N" is the highest region number.

3) When the final merge (after all input data is located in theworkspace memory) is done, and no partial merge has occurred, stringnumbers are assigned as a group to all rows in the highest region, thenall rows in the second highest region, and so on until all non-emptyregions have been assigned. This condition is depicted in FIG. 21 inwhich the final merge is indicated by "FM".

4) When the final merge is done, and a partial merge has occurred, theprocedure is more complicated. It should first be pointed out that:

a) Only 1 partial merge will ever be done.

b) It will always be from the second highest region to the highestregion.

c) No more merges will be done into the highest region (it is full).

The following procedure is followed. First, string numbers are assignedas a group to all but the last row of the highest region. Then they areassigned as a group to the rows in the second highest region thatremained immediately following the partial merge. Next the last row ofthe highest region is assigned a string number. Then the remainingunassigned rows in the second highest region are assigned as a group.Finally, any remaining non-empty regions are assigned as a group indescending order of region.

In FIG. 22, which shows the string number assignment for an optimalmerge after a partial merge, data is represented as follows:

"pppppp"=result of the partial merge.

"aaaaaa"=rows added to second highest region after the partial merge.

"bbbbbb"=rows remaining in second highest region immediately followingthe partial merge.

"++++++"=other filled rows.

Rebound Sorter Enhancements

The least significant bit of the tag byte (called the "order" bit) ofeach record entering the sorter is set to 1.

The order bit is used to preserve the order of records with the samestring number. The order bit is manipulated as follows (remember itenters the rebound sorter set to 1):

a) If the processing element decides to pass vertically or is alreadypassing vertically:

1) The tag byte passed "down" (the one that was coming down) has itsorder bit set to 1.

2) The tag byte passed "up" (the one that was coming up) has its orderbit passed unchanged.

b) If the processing element decides to pass horizontally or is alreadypassing horizontally:

1) The tag byte passed "down" (the one that was coming up) has its orderbit set to 0.

2) The tag byte passed "up" (the one that was coming down) has its orderbit set to 1.

c) The tag byte passed from IERS "N-2" to IERS "N-2+N-1" has its orderbit set to 1.

The stability of this method is shown by an example in which it isassumed that there are 2 duplicate records. For this discussion, we willassume that duplicate records are ones with completely equal keys andequal tags.

The first record enters the ERS and mixes with other records until thesecond record enters the ERS. In order to meet the second record, thefirst record must turn up, getting its order bit set to 1. Since the ERSis sorted, the second record will proceed straight down to meet thefirst one, keeping its order bit set. The 2 records will compare exactlyequal, including the order bits, and thus will continue to passvertically. In the absence of additional duplicate records, theduplicate records will continue up to pass larger records, or the topone will turn down (which clears its order bit) and meet the nextduplicate record coming up (which still has its order bit set). Thiswill cause them to preserve their current positions in the ERS untileither passed by a larger record, another duplicate enters, or the ERSis flushed.

Now it is only necessary to show is that a new record entering anexisting group of duplicates will fall to the bottom of the group. Asthe new record encounters each record in the existing group ofduplicates, the new one is coming down and the old one is coming up. Arecord coming up always has its order bit set. A record coming downbeing passed vertically always has its order bit set as well. This meansthat the new record will continue to pass vertically down the columnuntil it reaches the bottom of the group.

From the above it can be concluded that the rules can be changed toleave the order bits unchanged when passing vertically. This would allowrecords turned down to keep their order bits set to 0 if they continueto go down, but a record which is turned down never continues down sinceeverything below it is larger than it. This would simplify the logic inthe PEs.

By the indicated use of the order bit, records that might exchangeposition are forced to preserve their initial order.

Data Integrity Features

In cooperation with external hardware, data passing through the sortaccelerator is checked for corruption from the point where data entersthe device until it exits. In addition, when sorting or merging records,the order of output records is checked to ensure proper operation of thesort accelerator device and proper input record ordering for merges.

Error Detection Code Parity

Data paths outside of the sorter are checked using byte parity. Byteparity is a scheme where 8 bits of data are represented by a 9 bitvalue. The extra bit carries a parity value, calculated at the source ofthe data. Parity checking is accomplished by re-calculating the parityvalue from the data and comparing the newly calculated value with theparity bit. An inequality indicates an error in the data.

The sort accelerator 10 is supplied with data and parity from the hostsystem. As data enters, the parity is preferably checked. A errordetected at this point is classified as a system parity error, and willbe reported as such.

Parity protection of the data path from the system bus interfacecontinues to the rebound sorter. As data enters the rebound sorter, dataparity is again checked, with errors reported as internal data errors.

On output, the rebound sorter generates data with parity. When this datareaches the system bus interface, its parity is checked, withdiscrepancies reported as an internal parity error. The parity value isregenerated at the system bus interface and passed with the data to thehost system, allowing the host to check data parity as desired (parityis regenerated at the system bus interface to provide parity for datafrom the sort sequencer).

Checksums Calculation (FIG. 23)

Within the sort accelerator, data integrity is ensured by using a twolevel checksum scheme. Checksum protection is a method of errordetection where a checksum is calculated over multiple bytes. Thechecksum is later checked by re-calculating the checksum and comparingit to the previously calculated checksum. A checksum is calculated usingthe following formula:

    for i=0 to n, checksum=f(checksum, data i!)

As each record enters the sorter, a record checksum is calculated. Thefunction (f) for this checksum uses the parity of the data byte, using a"PAR" block as hereinafter described in connection with FIG. 23, toselect either the checksum or a bit reversed version of the checksum tobe added modulo 256 to the data byte. A "REVERSE BITS" operation isperformed on a byte which swaps the most and least significant bitswithin the byte, the next most and least significant bits and so on, sothat all bits have a different position.

These record checksums are added together modulo 256 to create achecksum over a group of records.

Records exiting the sorter have checksums similarly calculated. Theinput and output checksums are then compared after the sorter hasprocessed a group of records and has the remaining records flushed.

The simple summing of record checksums creates a function withassociative properties: the order the record checksums are summed haveno effect on the resulting checksum. This is important as the primaryfunction of the sorter is to reorder records.

The calculation of the Record Checksum is not associative.

If byte n of two records are swapped between the two records, the sum ofthe two record checksums will be different.

An example of a swapped bytes is as follows:

    ______________________________________                                        Record A      Record B        Sum of                                          data  par    checksum data  par  checksum                                                                             Rec. cksums                           ______________________________________                                        Input:                                                                        0x61  0      0x61     0x31  0    0x31                                         0x62  0      0xC3     0x32  0    0x63                                         0x63  1      0x26     0x33  1    0xF9   0x15                                  Output:                                                                       0x61  0      0x61     0x31  0    0x31                                         0x32  0      0x92     0x62  0    0x93                                         0x63  1      0x2C     0x33  1    0xFC   0x28                                  ______________________________________                                    

This checksumming scheme can check for two types of errors in the sorter18: single bit failure in the data path (including storage elementfailure) and the improperly controlled swapping of bytes between records(PE control failure).

The checksum operation is implemented using logic circuitry of FIG. 23which develops a "EDCNEQ" signal on a line 270 which is connected to theoutput of a "NEQ" comparison circuit 272 which compares the output oftwo sections. An upper section as shown in FIG. 23 includes an outputlatch 273 which is connected to the output of an adder circuit 274, oneinput of the adder circuit 274 being connected to the output of thelatch 273.

The other input of the adder circuit 274 is connected to the output ofan AND circuit 275 which has one input connected to the "EREC" line 160.The other input of the AND circuit 275 is connected to the output of alatch 276 which is connected to the output of an adder circuit 278. Oneinput of the adder circuit 278 is connected to the output of amultiplexer 280 which has one input connected to the output of the latch276 and which has a second input connected to a "REVERSE BITS" block281. Block 281 performs the aforementioned bit reversal of a byte.

The multiplexer 280 is controlled by a "PPAR" block 282, the input ofwhich is connected to a "IBYTE" line 283 (bytes entering the reboundsorter) the "IBYTE" being also connected to a second input of the adder278. The second lower section of the logic is substantially identical tothe first, corresponding elements being indicated by primed numbers. Thedifference is that the line 283' is an "OBYTE" line (bytes exiting therebound sorter) whereas the line 283 is an "IBYTE" line.

Sort Order Checker (FIGS. 24-28)

As records exit the sorter, they enter a special storage element whichholds one record. This storage element behaves like a shift register,allowing comparison logic examining the input and output of the storageelement to verify that records leaving the sorter are properly ordered.The comparison logic is similar to the processing elements used in thesorter with the exception of the output. The output of the comparisonlogic is a signal which indicates that a decreasing (increasing for"descending" sorts) sequence of records has been detected (a sort ordererror).

The sort order checker is also used to provide a "tie" bit indicatingthat a record in the output string has the same key as the followingrecord. The "tie" bit is the least significant bit of the tag byte onoutput from the rebound sorter. User software can utilize this "tie" bitto perform sorts on key fields which exceed the capacity of theaccelerator 10, as described in a subsequent section, or to aid inpost-sort processing of duplicate records. The sort Accelerator usuallystrips the tag byte from records before outputting them, but it can beinstructed to leave the tag bytes in the records if the user wishes.

The sort order checking operations are implemented by logic circuitryshown in FIGS. 24-28. Circuitry as shown in FIG. 27 includes an outputline 286 on which the aforementioned "SRTERR" signal is developed. Line286 is connected to the output of a latch 288 which is coupled to theoutput of an AND gate 289 having one input connected to a "-RESET" line290. The other input of AND gate 289 is connected to the other input ofan OR gate 282 which has one input connected to a line 286 and a secondinput connected to an AND gate 294. AND gate 294 has inputs connected tolines 295 and 297 which respectively receive "CHKING", and "EQUAL"signals. The third input to AND gate 294 is connected to a multiplexer.One input of the multiplexer is "A<B", the other is "A>B". Themultiplexer is controlled by an OR gate whose inputs are ASCENDING andETAG.

The circuit of the sort order checker further includes logic circuitryas shown in FIGS. 24, 25, 26 and 28. In FIG. 24, a comparator circuit298 is provided which develops the aforementioned "B<A" signal on theline 296 and also develops "A=B" and "A<B" signals on lines 299 and 300.One input of the comparator 298 is connected directly to a "UR0" line302 while the other input of comparator 298 is connected to the line 302through two cascaded IERS elements 303 and 304. These form theaforementioned special storage element which holds one record block.

In FIG. 25, the "CHKING" signal on line 295 is developed by a latch 306which is connected to the output of an AND gate 308 having one input towhich a "-NEWSTREAMOUT" signal is applied on a line 309, the"-NEWSTREAMOUT" signal being the inverse of the "NEWSTREAMOUT" signaldeveloped by the pipeline circuitry shown in FIG. 11. A second input ofthe gate 308 is connected to the output of an OR gate 310 having oneinput connected to the "EREC" line 160 and having a second inputconnected to the line 295.

In FIG. 26, the "EQUAL" signal on line 297 is developed at the output ofa latch 312 which is connected to the output of an OR gate 314 havingone input connected to the "EREC" line 160 and having a second inputconnected to the output of an AND gate 316. One input of the AND gate316 is connected to the "A=B" line 299 (FIG. 24) while the other isconnected to the line 297.

FIG. 28 shows the circuitry used to develop the "TIED" signal which isdeveloped on a line 318 connected to the output of a latch circuit 319which is connected to the output of an AND gate 320 having inputsconnected to the "CHKING" line 295, the "ETAG" line 174 from the circuitof FIG. 10 and the "EQUAL" line 297.

Exceeding the Sort Accelerator Capacity

The accelerator 10 may be used even when the available workspace memoryis not enough to sort records in a user's file. Algorithms are providedfor dealing with two very common cases. The first case is when notenough workspace memory is available to sort the total number ofrecords. The second case is when the record and/or key length exceedsthe maximum allowed by the sort accelerator.

Exceeding the Available Workspace Memory

When there is not enough workspace memory available to sort the userrecords, the user may use what is called an "external merge." Thismerely means that the user must manage the record storage for the inputand output of each merge pass required to complete the sort operation.

To begin with, the user uses the sort accelerator 10 to sort as manyrecords as will fit in the available workspace. It is not necessary tocompute this number. Records are simply fed to the sort acceleratoruntil an indication is provided that it is full. Each of these sortedstrings of records must then be placed in storage available to the user(they may be written to disk, for example). When all of the records havebeen sorted, or 16 sorted strings have been created, the user thenmerges the sorted strings using the sort accelerator 10 as a merger.This can be done with buffered input and output, and thus requires thatvery little memory be available to the sort accelerator. The sortedstring resulting from this merge can again be placed in storage. Thisalgorithm can be used iteratively, limited only by the availability ofadditional record storage.

Exceeding the Maximum Record Length

There are 2 cases where the record length exceeds the maximum allowed bythe sort accelerator.

The first, and simplest, case is when the total record is too large, butthe key plus a pointer to the original record will fit. In this case,new records are input to the sort accelerator made up of the originalkey followed by bytes containing a pointer to the original record. Whenthe sort has been completed, the user merely processes the sortedrecords, using the pointer to access the original record contents.

The second case occurs when the record is too large, and the key plus apointer to the original record will not fit. In this case, the useragain inputs new records, but this time the user only puts in as manykey bytes as will fit, leaving room for the pointer at the end. Whensorting with these truncated keys, a request may preferably be that thetag byte be output so the user can examine the "tie" bits.

In many cases, the truncated keys thus presented will be unique enoughto determine the sorted order of the records. It is merely necessary toexamine the "tie" bits to locate groups of records with duplicatetruncated keys. This group of records can then be further sorted byusing as the new key, as many of the remaining key bytes from theoriginal record as will fit. This process may be iterated on all groupsof records with duplicate truncated keys until there are no duplicatekeys reported, or all key bytes have been processed.

It should be pointed out that this process can potentially run fasterthan using a sort accelerator with a larger maximum record size. This isbecause the speed of the Sort Accelerator is proportional to R * N *LOG16(N) where R is the record length presented to the Sort Accelerator,and N is the number of records. By using a smaller value of R (asrestricted by the maximum R), the first pass sort of the records runsfaster. Subsequent passes (if any) will very likely use a much smallervalue of N to resolve any duplicates, which will be much faster.

It will be understood that modifications and variations may be effectedwithout departing from the spirit and scope of the novel concepts of theinvention.

What is claimed is:
 1. A sorting system for serially processing recordscomprising:input means for serially supplying groups of input records;sorting means for serially receiving said groups of input records fromsaid input means and serially outputting more than N+1 of said inputrecords in sorted order; said sorting means comprising a plurality of Nprocessing elements and a plurality of storage elements, each of saidplurality of N processing elements being connected to at least two ofsaid plurality of storage elements, each of said N processing elementsbeing able to concurrently transfer two of said records to be sorted totwo of said storage elements connected thereto, each of said storageelements comprising a RAM section and variable length counter means tocontrol indexing of said RAM during sorting of said input records.
 2. Asorting system for serially processing records comprising:input meansfor serially supplying groups of input records; sorting means forserially receiving said groups of input records from said input meansand serially outputting more than N+1 of said input records in sortedorder; said sorting means comprising a plurality of N processingelements, a plurality of storage elements, and a merging means, each ofsaid plurality of N processing elements being connected to at least twoof said plurality of storage elements, each of said N processingelements being able to concurrently transfer two of said records to besorted to two of said storage elements connected thereto, each of saidstorage elements comprising a RAM section and variable length countermeans to control indexing of said RAM during sorting of said inputrecords, said merging means for merging intermediate groups of saidinput records when said sorting means sorts more than N input records.